The other day I spent like 4 hours removing unneeded stuff from my PC. It took me this long because I had several old files like unused VirtuaBox kernel extensions (I remember installing them when trying to learn some WebOS programming), numerous useless plugins all over the computer, other kinds of extensions, trash, etc. I googled each one of them to make sure I don't delete important things by mistake.
This is a list of things/places to check so in the future I don't have to google everything again. :)

First, EtreCheck is a tool I like it very much because is not offensive or intrusive as Onyx or similar tools, unlike CleanMyMac 2 it has no ads (CleanMyMac is crap). EtreCheck is free, relatively fast and open source.
EtreCheck attempts to just report the facts, without making any judgements or recommendations. Debugging is not a task that can be automated. It needs humans, ...
Just run the app and it will tell what extensions, plugins, etc is loaded in the system. These are good places to start to look for.